For the second time in a month, a deeply divided Ukraine casts ballots in a runoff election between presidential candidates who have increased their hostile rhetoric since their last fraud-marred vote, raising fears of violence no matter who wins.

Opposition candidate Viktor Yushchenko — whose face still is badly scarred from dioxin poisoning this fall that he blamed on authorities — has emerged as the front-runner in Sunday's election, building on the momentum of round-the-clock protests launched by his orange-clad supporters after his rival, Prime Minister Viktor Yanukovych, was declared the winner of the Nov. 21 runoff.

That balloting was later voided by the Supreme Court, which found there was massive vote-rigging and ordered Sunday's new vote.

On Thursday, Ukraine's Security Service denied any involvement in the poisoning of Yushchenko, saying in a statement on its Web site: "It has no relation with the worsening" of his health.

"The Ukrainian Security Service did not obtain a single official document that could provide ... a basis for the establishment of the time or the place or the fact of the candidate's poisoning," the statement said.

In an interview with The Associated Press last week, Yushchenko said he was probably poisoned at a Sept. 5 dinner with security service chief Ihor Smeshko and his deputy, Volodymyr Satsyuk. Both denied any involvement.

Yushchenko has recovered enough to return to the campaign trail and lead the mass protests dubbed the "orange revolution," likening them to the mass movements that swept aside the Berlin Wall and signaled the end of Communism in eastern Europe.

Yanukovych has warned that his opponent cannot win over Ukraine's densely populated, Russian-speaking east, saying a Yushchenko victory would only be acknowledged by part of Ukraine.

But the giant street protests and the annulment of Yanukovych's victory have weakened the prime minister, and opinion polls show him likely to lose.

He has been abandoned by his principal backer, outgoing President Leonid Kuchma, deserted by his top lieutenants and forced to reinvent himself as an opposition figure — even though he runs the government.

The election will be monitored by at least 8,000 observers and will be conducted under recently changed electoral laws tailored to prevent fraud.

But Yanukovych has already alleged that changes to regulations over voting at home and absentee ballots will disenfranchise millions.

Yushchenko told a roaring crowd of supporters Wednesday to be on guard against plots to disrupt the election.

In addition to the bizarre poisoning plot, the political crisis has revived echoes of the Cold War, with the Yushchenko leaning toward the West and Yanukovych getting his backing from the Kremlin, which sees Ukraine as its main satellite.

Yushchenko's supporters believe he has a chance to nudge Ukraine closer toward the European Union.

Yanukovych's backers fear severing this nation's historic, cultural and linguistic ties with neighboring Russia, and discrimination by the Ukrainian-speaking, nationalistic western part of the country.

Russian President Vladimir Putin has taken issue with Western and particularly U.S. activism in Ukraine. Yanukovych has accused Yushchenko's campaign of being financed by the United States, a charge that he has denied.

This week, however, the Kremlin appeared to hedge its bets, with Putin saying he could work with Yushchenko if he wins.

"Ukraine is moving out of Russia's backyard, and with that will come a pragmatic relationship," said Darya Glushenko, a political analyst with the Kiev-based International Center for Policy Studies.

On Thursday, Ukraine's foreign minister said Russia should view the possibility of Ukraine's joining NATO as a positive step.

Konstantin Grishchenko told a news conference in Moscow that the two countries have such close political, economic and historic ties that, were Ukraine to join the military alliance, Russia could be assured that NATO would not harm it.

"We are so close ... that it's impossible to imagine the situation where Ukraine joins some organization which acts against Russia," he said.

Grishchenko, who also met with his Russian counterpart, said no matter who wins Sunday's presidential election in Ukraine, relations with Russia will remain unchanged.

Russia has long viewed with distrust NATO's expansion into eastern Europe.